MY kids can tell when it is a bad day for smog in Beijing. It seeps into the hall of our apartment building and they can taste it in the air – it’s tangy, like an airborne mixture of oil and gas.

Usually, we take a quick look out of our front window on to Jianguo Avenue, the thoroughfare that traverses central Beijing and is permanently full of cars, to test whether the air is clean enough for us to see the skyscrapers across the street. Today – when pollution readings were at such hazardous levels that they had risen above the scale used to measure them – all we could see were clouds of pollution.

The smog was so thick that more than 50 flights were cancelled at Beijing Capital International Airport, causing chaos ahead of Chinese New Year, when city-dwellers travel to see relatives. Despite warnings from the authorities not to venture outside, we decided to don our pollution masks and take a quick trip to the shops. The warnings are increasingly common but are not observed by most residents who cannot afford – or stand – to spend days confined to their homes.

Our masks are white Totobobo masks, from Japan, and we each have one tailor-cut to fit our face. My 10-year-old son Fred likes to wear his so he can pretend he is the character Bane from the Batman film The Dark Knight Rises. He does a great impression of the actor Tom Hardy’s respirator-garbled voice. “It is easier to crush Gotham than it is to solve the pollution problem in Beijing!” he says. My daughter Anna, four, finds her mask annoying but she knows it is important to put up with it.

On the street in the Sanlitun shopping district, we spotted a salesman with a big box of masks doing a brisk trade. There is a limited number of official suppliers selling pollution masks that are recognised as being of high quality, and as most are running out of supplies, the trade in lower-quality fakes is booming.

A trip out in the smog is rare for our family: my partner and I usually don’t let our children go out when the readings are very high. But “very high” is also a relative term. We have become aficionados of the China Air Quality mobile phone app, which displays data collected by the US Embassy in the Chinese capital.

The Air Quality Index it uses gives a reading according to how many particulates of less than 2.5 micrometres in diameter (PM2.5), which are believed to pose the largest health risks, are in the air.

The AQI was at 500 for much of the day – the highest and most hazardous level on the scale, suggesting that the air quality may have been even worse. The World Health Organisation recommends 20 as a healthy level.

These tiny PM2.5 particles are of particular concern “since they are small enough to directly enter the lungs and even the blood stream”, according to the US Embassy’s website, and they are known to cause lung cancer, bronchitis and asthma. Thankfully, our children have not suffered any ill health as a result of exposure to pollution, but our neighbours’ kids suffer from asthma and repeated cases of bronchitis. We insist that our children wear their masks when they go out, and we also make sure they spend at least a month out of Beijing every year in a clean-air environment. I developed mild asthma because of the smog a few years ago, which thankfully appears to have ebbed, though I do worry that it will come back.

For years, the Chinese government insisted on referring to the smog as “fog” and released unrealistically low air-quality readings. Now, the official data has become more reliable and the government calls it “smog”, but this has only left local people even more concerned that the situation is far worse than is reported. The World Bank reckons that 16 of the world’s 20 most-polluted cities are in China. The China Air Quality app gives subscribers the opportunity to see which cities are the worst every day. While Beijing feels as though it should be top of the list to us, it is often far down the list. Late in the evening, Shijiazhuang, one of the worst serial offenders, was “beyond index”, while Sanya in the south, on Hainan island – known for having the cleanest air in China – displayed a reading of just 22.

Coal-fired power stations account for more than 70 per cent of China’s energy production, while nearly 20 million vehicles were sold across the country last year, making it the world leader in car sales. Sometimes it seems as it they are all driving up and down outside our window.

Many of the new cars should have low emissions, but the problem is that the sulphur content of vehicle fuel is very high. In Beijing, personal and public vehicles contribute the largest portion of sulphur dioxide, nitrogen dioxide and other pollutants to the atmosphere. Heavy, polluting industrial plants were moved outside the city, but only as far as the foot of surrounding hills, where the smog does not disperse easily and often blows back in to choke us.

Ma Jun, a Beijing-based environmentalist and founder of the Institute of Public and Environmental Affairs, told the Global Times newspaper there were more steps the government could take to ease the smog with immediate effect. “Dust from construction sites can be decreased if the government puts more efforts into supervision, and factory [emissions] should be improved,” said Ma.

Authorities said they would take emergency measures to curb the pollution levels in Beijing, halting production at 103 high-emissions companies, while government agencies and state-owned firms were ordered to cut vehicle use by 30 per cent before Thursday, when it is believed that the air should begin clear. – until the next time the reading goes “beyond index”, that is.